It has long been known in the prior art to provide apparatus for purposes of effecting the grinding and pulverizing of certain materials. More specifically, the prior art is replete with examples of various types of apparatus that have been used to effect such grinding of a multiplicity of materials. Coal is one such material wherein there is a need that it be ground to a particular fineness in order to render it suitable for the use in, for example, a coal-fired steam generating power plan.
One particular coal pulverizing apparatus, which is to be found in the prior art, is a roller mill. An exemplary roller mill with an integral classifier is shown and described in U.S. Pat. Nos. 4,640,464 and 7,028,847. An exemplary roller mill 10 having an integral classifier 12 is depicted in FIG. 1. As is typical of such a roller mill, the mill includes a plurality of grinding rollers 14 that toll along a grinding ring 16 for pulverizing the material to a desired particle size, which defines a grinding zone 18. A blower (not shown) generates an upward airflow 20 that draws fine particles upward through a grinding chamber 22 to the classifier 12 disposed above the mill housing 24 and in fluid communication therewith. The classifier has a centrifugal-type separator 25 that separates the oversize particles from the finer particles. The finer particles pass through the classifier through an output duct or port 26, as shown by airflow 28, while the oversized particles fall back down to the grinding chamber 22 and rollers 14 for continued grinding, as shown by downward particle flow 30.
It has been found that when the roller mill 10 is used for grinding fine particle sizes, a significant portion of the oversized particles rejected by the classifier at the top of the mill can still be quite fine. These rejected oversized particles returning back to the grinding zone 18 along the mill housing 24 in the downward particle flow 30 are subject to the airflow 20 flowing upward from the grinding chamber 22 to the classifier 12. This upward airflow 20 can entrain the rejected oversized particles quite easily and recirculate them back to the classifier, as shown at 34, where the classifier will reject the recirculated oversized particles again. This situation creates an internal recirculation of the oversized particles between the grinding chamber 22 and the classifier 12. As a result, a significant amount of rejected particles may never make it back to the grinding area to get further reground. These rejected oversized particles are thus suspended in the air stream, causing a pressure drop, which reduces mill capacity, and thus lowers the mill's efficiency. Therefore, there is a need to reduce or eliminate this recirculation phenomenon of these rejected oversized particles from recirculating between the grinding chamber and the classifier chamber without being reground.
One prior art separator apparatus disclosed in U.S. Pat. No. 5,279,466 shows a roller grinding mill having a classifier that redirects the oversized particles to a return path that is different from the upward flow of material from the mill so that the output from the mill is substantially free of being interfered with its movement by returning the oversized particles through a conduit or pipe back to the material feed or grinding chamber. As shown in this prior art, the separator apparatus has a rotor with a plurality of blades that centrifugally directs oversized particles to the outer wall of the separator apparatus. The oversize particles fall within an internal passage defined by the wall of the roller mill and the outer wall of the classifier. The particles are then funneled to an opening and/or a conduit that may direct the oversized particles to the rotary feeder or back into the grinding chamber. While removing the oversized particles from the upward airflow, the funneling of the oversized particles to an opening or conduit is susceptible to potential clumping of the particles and/or clogging of the opening and conduit. Furthermore, the depositing of the collected oversized particles concentrated at specific locations along the grinding ring will result in a non-uniform bed depth due to the localized depositing of the return of oversized particles, resulting in a decrease in grinding efficiency and/or increase in mill grinding noise. Furthermore, the device provides an external return path for the oversize particles that requires a relatively complex and costly oversized classifier housing, chutes, and conduits for accommodating the return path for the oversized particles.
What is needed, therefore, is a means for providing a simple return path separate from the central flow of fine particles from the grinding chamber to the classifier back to grinding zone, whereby the oversized particles rejected by the classifier are distributed in the grinding chamber in a less concentrated manner around the grinding ring to provide a more efficient grinding process.